Today is all about Facebook's big $100 billion IPO. But how did the social media titan get so titanic to begin with? Harvard Business Review's David Rock explains that Facebook is so far ahead of human practice, it actually hacks our brains.

While Facebook's rise took many by surprise, its success was little surprise to the hundreds of researchers who study social interactions in neuroscience labs across the country. Over the last decade, these neuroscientists have uncovered some unexpected quirks of the brain, that all link to one big idea: we are far more socially oriented, at the level of brain structure and systems, than we account for in daily life.

Why does this matter? It certainly matters to Google, or to any organization wanting to get people's attention. Yet this insight also has a dark side that deserves some airtime too.

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Here's how social the brain is: the brain network that is always on in the background is a region involved in thinking about yourself and other people. This network is so ubiquitous it has been labeled the "default network." When not doing anything else, the brain's favorite pastime is to think about people. We actually turn this region down when we do any active processing, such as doing math. One study showed that inactivity for just two seconds switched the default network back on.

Many studies have emerged in the last few years about the importance of human social interactions to our well-being. We know that social rewards light up the brain's reward circuits more than non-social rewards, and that social threats, such as feeling lonely or ostracized, light up the threat center more than non-social threats. We've even seen that social pain, like being left out of an activity, lights up the same regions as physical pain. And that taking Tylenol can reduce social pain more than a placebo.

These types of findings explain the success of social media. We're giving people something that deeply excites the brain in highly condensed form, which keeps them coming back. After all, the brain is built to minimize danger, and maximize rewards, and in a modern society with few real dangers, we focus on the most rewarding activities that take the least effort (minimizing effort is also seen as a reward).

Here is the seed of the problem. Social media can be so rewarding, that it overwhelms our ability to focus on other things. Our brain has terribly weak circuitry for inhibiting impulses, especially impulses that look delicious. Like our limited ability to do complex calculations in our heads, impulse control is a limited resource that tires with each use. For decades, food marketers have used this poor impulse control against us, to the point that there are now literally more people overweight than starving in the world, in large part due to empty calories that are all too readily available. Our minds may be going the way of our waistlines, as a result of "empty neural calories": fodder for the brain that stimulates but doesn't fulfill.

There is a circuitry for "seeking" and a circuitry for "liking." The liking response settles down the excitement of the seeking circuitry. Without the liking response, we're like the rat pressing the level over and over to get a little dopamine hit, forgetting all about food and rest.

The circuitry activated when you connect online is the seeking circuitry of dopamine. Yet when we connect with people online, we don't tend to get the oxytocin or serotonin calming reward that happens when we bond with someone in real time, when our circuits resonate with real-time shared emotions and experiences. On Twitter, you won't feel satisfied the way you might if you chatted in person with 50 people at a conference.

An overabundance of dopamine - while it feels great, just as sugar does - creates a mental hyperactivity that reduces the capacity for deeper focus.

If your job is to stay "high" all the time, like a reporter on TMZ, then a hyperactive state of mind isn't a problem. But forget about trying to focus, think deeply, or perhaps learn something. A study by psychology students at Covenant College found that the more time young people spend on Facebook, the more likely they are to have lower grades and weaker study habits. Heavy Facebook users show signs of being more gregarious, but they are also more likely to be anxious, hostile, or depressed. Almost a quarter of today's teens check Facebook more than 10 times a day, according to a 2009 survey by Common Sense Media.

Throughout history, whenever a new technology emerged that dramatically changed how people interacted, it took time for our human practices to catch up. When the automobile first came out, people would drive at all speeds, in every direction. Eventually road rules and speed limits were put in place, and the world was a safer place. Facebook per se is not evil, just as cars are not evil. However our relationship with the automobile is safer overall with some rules in place, combined with good driver education.

Perhaps we need to start thinking about some road rules and speed limits for social media use. This is certainly important for our kids, whose self-regulation circuits are just forming. However given that self regulation is not one of our strong suits in modern society, perhaps we all need a better understanding of the impact of this new tool on our very ability to think.